Gas or vapor arc discharge lamp



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INVENTOR BY /ff/ @fw ATTORNEY Patented Get. 19, 1937 iJNiTiizi) STATES PATENT y OFFICE 2,096,704 Gas on varon Anc DISCHARGE LAMP y Laurence Burns', Revere, Mass., assigner to' Hy'- grade Sylvania Corporation, Salem, Mass., a

corporation of Massachusetts Application July 17, 1935, Serial No. 31,754

3 claims.

As pointed out in application Serial No. 24,991v

flied June 5th, 1935, one of the causes 'of loss of luminous -eiiciency in arcedischarge lamps is the tendency of the arc-flame to bow outwardly and away from the central longitudinal axis of the enclosing envelope. This tendency to bow is a function of the convection currents within the gas or vapor lling. I have'ound that the positional stability ofthe arc may be maintained' by proper proportioning of the length of the arc to the arc current or to the arc cross section. Ac-

cordingly another object of the invention isto provide an arc-discharge lamp which is designed to have a definite relation between the arc current or cross section andthe length of the arc.

Another object is to provide an arc-discharge lamp which operates with a relatively high pressure filling and which is -so designed as to produce a predetermined ratio of arc length to arc cross section at which the arc has maximum positional stability.

A feature of the invention relates to'an arcdischarge lamp of the type operating at relatively high pressure of the ionizable filling between electrodes arranged to strike an arc confined sub stantially to thecentral or axial portion of theA enclosing envelope, the eelectrical and mechanical parameters ofthe lamp being so chosen that the arc maintains vits central position with very little tendency te wiggle or positional instability.

A further feature relates to the novel organi--v zation, arrangement and relative proportioning of parts which goto make up an improved and highly efcient lamp of .the high pressure arcdiseharge type- Other features andadvantages not specifically l55 lamp embodying. features oi' ine-invention.

A ionizable medium or mediums such for example Fig. 2 showsa series of curves for explaining certain aspects of the invention.

Referring to Fig. 1 of the drawing there is Ashown in diagrammatic form, a lamp having an enclosing envelopeor main body portion I. f 5 substantially cylindrical shape and preferably of a refractory transparent material such as glass combustion tubing. The body vportion I is closed oli at the ends by concave cap portions 2 of a softer grade of 'glass and through which 'the lead- 10 in Wires 3, si are sealed preferably through the intermediary of softer glass beads 5; Supported on eachpair of lead-in wires is an electrode preferably formed of refractory material such as tungsten, molybdenum or the like provided with 15 a forward pointed or V-shaped section 6 joined to the lead-in wires through coiled or zig-zag sections '1, 8. As disclosed in application'Serial No. 24,991 the sections 1, 8 are preferably disposed within the angle subtended by the V-shaped section so that. the latter section'shields the sections 1, 8 from a migrating discharge. The en' tire electrode, or if desired only the V-shaped section, is coated with Jsuitable electron-emissive material such for example as theoxide or oxides of the alkaline earthsand the electrodes are so mounted that the points are in alignment preferably on the longitudinal axis of the envelope L If desired a shield 9 may be positionedin back of eachelectrode for purposes set forth in detail in application Serial No.'23,992 led May 29th, 1935.

'After being suitably'evacuated and degasied in 'accordance with accepted lamp procedure, the

enclosing envelope is provided with a. filling of an as a filling of mercury vapor and an inert gas such '35 as argon or mixtire of inert gasesto facilitate starting. If desired the lamp proper may .bev enclosed within an outer jacketeither evacuated, open to the air, or filled with a gas for the purpose .of conserving .the heat within the ionized fllling when the lamp is in use.

with the type of lamp described, when a suitable voltage is impressed across the electrodes, 'a

glow discharge starts in a similar manner to the starting of a glow from thermionically inactive electrodes, but the pasasage of this glow current serves to raise the electrodes to a temperature at which they function mainly as. thermionicemit-l ters. When thiscondition is attainedY the discharge between-g the' electrodes is arc-like in character and is characterized by a small electrode drop approximately' 1 5 volts. This preliminary ai'lc occurs in the argon -or other -inert gas lling d the heatof this arc gradually warms Athe en- 55 shrinks to a diameter smaller than thatV of the envelope l, becoming more and more constrictedas the density of the vapor increases. When the vapor density reaches a certain value for a given size of envelope the arc column will have a tendency to bow."outw-ardly from the central' portion of the envelope. When the lamp is used in a vertical position this bowing phenomenon is most pronounced at the bottom electrode and is probably`\ caused by the convection currents in the vapor and gas lling. 'When the lamp is in operation the arc flame temperature is of the order of 6000 -degrees centigrade, while the tem- `-perature of the envelope is of the order of 600 degrees centigrade. Because of this temperature gradient from the central part of the envelope to the wall, the vapor will tend to rise near the arc flame, and return again along the wall I. Any slight defect in the centering of the arc column will have an exaggerated effect on the convection currents, with resultant increased` tendency of the arc to bow or wiggle. The positioning of the electrode discharge surfaces along the central axis of the envelopereduces this tendency to positional instability of the arc. Furthermore in the high pressure stage of the lamps. operation` the discharge proceeds from a very restricted area o! each electrode, and with the shape of electrode shown in the drawing, this discharge is confined substantially to the points 8. As lthe vapor density is further increased, a point-is reached where the arc column begins to move around appreciably within the envelope I.

In some cases this positional instability reaches such an amplitude and'occurs atsuch a frequency as `to extinguish eventually the arc itself, that is, the column moves enough to break itself, lthis phenomenon alsobeing a function of the convection currents in the vapor.` I have i'oundthat in the lamp as described,'convection will take place smoothly in streamlines.- resulting in socalled lame11ar" flow when the lamp is designed to produce an arc columnhaving a certain ratio of arc length to arc current or to arccross section. I'have also found that the factor which determines the amount of positional instability in the arc column is not vapor density alone, but that the geometry of the discharge tube and the electrical constants of the discharge are very important. For example a relatively low power lamp such as a 400 watt input lamp, may be v quite stable at a density corresponding to atmospheric pressure; `but a higher power lamp, for

. example one of 2000 watts may exhibit decided wiggling or positional instability at half that density. On-.the other `hand I have found that for a constant density, a lamp may' have serious positional instability at' one arc current, for example 5 amperesmnd substantially negligible instability at a higher current, for example 1 0 amperes. After numerous experimentsl have vfound that the tendency to "wiggle" or positional instability decreases in proportion to the increase ofarc current, and is probably due to a widening 'of the arc column. Furthermore I have found that the longer the arc column a proportionately closing envelope and tle vapornlling so that the pressure of this vapor rises and the envelope greater current is required for a given arc stability. lior loperating pressures of the orderof an atmosphere I have found that approximately one-half ampere is required for each inch of arc length, andthis ratio with slight modification holds for even lower pressures down to as low as one-half atmosphere and as highy as one-and-onehalf atmospheres. Preferably however where the operating pressure of the ionlzable lling is materially less than one atmosphere the arc should have a diameter ofthe order of atleast 0.025 inch for each inch of arc length.

I have also found that for a given lamp size and with a given power input the luminous efilciency risesV very rapidly with increasing voltage untilv the vapor density reaches a certain value,

then remains relatively uniform throughout a` range of densities, and as the vapor density continues to rise beyond this range, the luminous eillciency fallsl relatively rapidly. Thus there are shownin Fig. 2 characteristic curves of two lamps showing the relation between the varia- Athe luminouseillciency remains -relativelyl uni- .lorm. As ,the voltage is.increased above 350 volts there is ,a sudden decrease in emciency and in the region where the efficiency starts to fall voltage is increased above 200 volts the eillciencyj..

drops on, and the arc wiggle becomes quite vpro-` nounced. In the case of the lamp represented by curve 1 lthe point A corresponded approximately to a vapor density of about 0.75 atmosphere while in the case of the lamp represented Iby curve 2,-the point A corresponded approxi mately to a vapor density of about 0.5atmosrapidly as indicated by the letterB, there is a I' pronounced tendency `or the arc to "wiggle or phere. The point ,B on' curve l correspondedto a vapordensity of about 1.0 atmosphere o rl ap proximately 0.002 gram per cubic centimeter,

while the point 'B' corresponded to about' 0.75

atmosphere. Generally therefore the lamps whether of the high or low power input.

type exhibit their highest luminous emciencies in theregion below that where the wiggle" becomes pronounced and in this region of high eillclency the arct current is of the 'order o! about 0.5 ampere per inch of arc length whereasin the region between the points A and C a closer approximation is .the relation between the arc length and the arc cross section which is about 0.025 inch 4diameter of arc to each inchk of arc length..

In the'particular lamp shown enclosing envelope l was apprordmately 19 inches mi in length and approximately 2% inches in internal diameter. vThe arc length L was approximately 17% inches and the lamp was arranged to be operated at approximately 2000 wattspower. input from asuitable source Il.

When the lamp was operating at maximum lu- Ilv minous Aeinciencsn the arc width D was approximately 0.8 inch and the arc current was approximately 8.5 amperes, the vapor density of the mercury vapor lling being approximately 0.7 atmosphere at this point of maximum luminous efciency. It will be understood ofcourse that the invention is not limited to any particular size of lamp vor to am particular power input as long as the ratios between arc length and arc current or between arc length and arc cross section are substantially those described above. Accordingly. Various changes and modications may be made' herein without departing from the spirit and scope of the invention.

What is claimed is: 1. In combination a high pressure vapor arc lamp comprising an enclosing envelope having a lling of mercury vapor an'd a small percent of an inert conductive gas, a pair ot electrodes mount# 20 'ed at opposite ends of saidenvelope, a shield in back of each electrode to control the convectioncurrents within the vapor, and means to apply aooa'm.

a voltage across the electrodes to strike an arc having a ratio of arc length in inches to arc cross section in square inches of the order of 1.0/0.0005g- 2.'The combination according to claim 1 in 'v which the enclosing envelope is substantiallyV cy." lindrlcal and the electrodes are provided withj pointed opposing portions between which the arc column is localized, the said pointed portions being located on the centrallongitudinal axis of the envelope. f

3. In-combination, a high pressure vapor arc lamp comprisingan enclosing' envelope having a iilling of mercury vapor and a small percent of an inert conductive gas, a pair of electrodes mounted at opposite ends of said'envelope, a shield in back of at least one electrode to control the convection currents within the vapor,

and means to apply a voltage yacross the elc- 1 LAURENCE BURNS. 

